The invention relates in general to an assemblage of rods for generation of multipole fields to be employed for ion guide and mass analysis purposes, and more particularly to economical and precise construction arrangements thereof.
The transport of ions over some spatial interval is a functional description of an ion guide. Such an ion guide is an electro-optical device for confining the ion trajectories to a generally axial locus and that confinement is achieved through the influence of an appropriate electric multipole field distribution that returns a non-axially directed ion trajectory back toward the axis. The most common structural form for such a guide consists of a number, 2N, of metal rods arranged equidistant from a central axis. Opposite and/or 180xc2x0 phase shifted AC potentials are applied in common to alternate rods. The efficacy of the ion guide depends upon precise geometry of the rod assembly as well as the congruence of virtual source and exit apertures of the guide with the real apertures of devices between which the ion guide operates. In one system (a mass spectrometer), an ion source is disposed spaced apart from a mass analyzer with the ion guide therebetween. Separation of the ionization and mass analysis procedures and devices permits optimization of these procedures and hardware subject to the efficiency of the ion guide.
The prior art has approached ion guide construction through support of the array of rods with at least a pair of axially spaced support assemblies having holes for retaining the relative positions of the rods and also for providing the desired common electrical contact of alternate rods. These prior art support assemblies typically include a ceramic insulating ring having holes through which the rods pass, to define the relative disposition of the rods. Metal rings are secured to the opposite faces of the ceramic insulator and each metal ring forms a common electrical contact with one corresponding sub-set of alternate rods while maintaining electrical isolation from the other sub-set of rods. Such arrangements are described in U.S. Pat. No. 6,329,654 B 1 and in U.S. Pat. No. 5,852,294.
It is desired to achieve a precise geometry for an ion guide with a simplified assembly. This is obtained with a support collar construction employing an insulating ring having axial extent sufficient to accommodate two axially spaced peripheral grooves on the outer azimuthal surface. An inner azimuthal surface has rod conformal arcuate surface portions formed therein to locate each rod. Each groove is characterized by a set of radially directed holes azimuthally spaced 2xcfx80/N radians for an assembly of 2N rods. The angular positions for the hole set for one groove is staggered xcfx80/N with respect to the other groove. Common electrical contact for one sub-set of N rods is realized by a conductor disposed within the groove, which contacts a rod through the respective radially directed hole.
The arcuate surface receives and constrains the outward radial locus of a rod. Electrical contact is established with a strong conducting wire captured in the groove and stressed such that when bonded through the holes to respective rods, there is an outward force on the rods balancing the inward constraining force of the arcuate surface portion against the rod, e.g., preloading the rod against the collar. As a result the ion guide assembly is a robust self aligned structure and is characterized by an aperture limited only by the rods themselves.